Wavemeter for microwave energy



March 1, 1955 E. L. GINZTON ETAL WAVEMETER FOR MICROWAVE ENERGY OriginalFiled Jan. 29, 1943 Z "w. in 2 MflG/VET/C FLUX INTENS/ T Y E "a /$92 1-:L. $44 ISBURY ATTORNEY United States Patent WAVEMETER FOR MICROWAVEENERGY Edward L. Ginzton, Menlo Park, and Frederick L. Salisbury,Redwood City, Calif., assignors to The Sperry Corporation, a corporationof Delaware Original application January 29, 1943, Serial No. 474,016.Divided and this application June 30, 1949, Serial No. 102,276

Claims. (Cl. 250-39) This invention relates to wavemeters for precisedetermination of the wavelength and frequency of microwave energy.

This application is a division of our application Serial No. 474,016,filed January 29, 1943, issued as Patent No. 2,503,256.

An object of the present invention is to provide wavemeters of minimumenergy loss and of extremely high Q, to permit very criticaldetermination of wavelength or frequency.

It is a further object to provide extremely accurate wavemeterarrangements of very high stability, such that a calibration of theinstruments, once made, remains accurate indefinitely and is notdisturbed by wear of contacting surfaces.

The present invention comprehends the provision of a precision resonantcavity structure of low loss and of apparatus cooperating therewith forindication of the response to microwave energy supplied thereto througha connecting wave guide, the resonant cavity structure be ing variablethrough precision calibrated adjusting means to permit it to be adjustedfor maximum indicated response and to provide a reading indicative ofthe wavelength or frequency of the energy.

The hollow cavity structure has a cylindrical interior bounded byelectrically conductive surfaces, and one of the end boundary surfacesis movable toward the opposite end boundary surface for varying theenclosed volume of the wavemeter cavity, and accordingly varying itsnatural period or resonance frequency. This movable end surface elementis provided with multiple quarterwavelength transmission line sectionsof alternatly low and high characteristic impedances, arranged toprovide an effective peripheral connection between the movable boundaryand the adjacent walls of the chamber without requirement of an actualdirect connection such as a friction joint.

The above objects and general description of the invention will beillustrated and further objects will be made apparent in the ensuingdetailed description of the invention, given in connection with theaccompanying drawings, wherein:

Figs. 1 and 2 are longitudinal and cross sectional views of onewavemeter embodiment of the present invention;

Figs. 3, 4, 5 and 6 are mode charts illustrating suitable electric andmagentic field distributions for the wavemeter illustrated in Figs. 1and 2; and

Fig. 7 illustrates a larger-cavity wavemeter embodiment of thisinvention.

Referring now to Figs. 1 and 2, a hollow spool-like body member 76having a cylindrical bore 77 and end flanges 78 and 78 is provided withan energy input passage or aperture 96 at its left end. A plate 79 issecured as by screws 81 to the body member 76, and is provided with acentral boss threaded both internally and externally. A spindle 82 isformed with a section 83 threaded to mate with the internal thread ofthe boss in plate 79. A micrometer-type sleeve 84 is fixed to the outerend of the threaded section 83 of spindle 82, and is provided with anengraved scale 85 of angularly spaced engine divisions at its left-handend. I

A micrometer-type barrel 86 having a divided linear scale 87 forcooperation with scale 85 is rigidly fixed to body member 76. The pitchof the divisions of scale 87 corresponds to the thread pitch of threadedsection 83, so that the circular edge of sleeve 84 travels an axial dis-Patented Mar. 1, 1955 tance of one division of scale 87 per revolution.Thus, the divisions of the circular scale 85 correspond to precisesubdivisions readable along with the divisions of scale 87, in themanner well known in the micrometer caliper art.

A conductive end disc surface 88, effectively comprising one end wallor" the resonant chamber bounded by bore 77 and the integral left-handend wall of spool 76, is mounted on the inner end of spindle 82. A thinrightwardly extending cylindrical sleeve 89 is provided on the peripheryof disc 88. This sleeve 89 may be formed integrally with end disc 88.The spacing between the cylindrical sleeve 89 and the spindle 82 is muchgreater than the spacing between sleeve 89 and the bore 77, andcylindrical sleeve 89 is substantially one quarter-wavelength long for afrequency in the band over which the wavemeter resonates, so that afirst quarter-wavelength coaxial transmission line section of lowcharacteristic impedance is formed between cylindrical sleeve 89 andbore 77 and a second quarter-wavelength coaxial transmission linesection of appreciably higher characteristic impedance is formed betweencylindrical sleeve 89 and spindle 82.

A further unit including a disc-like portion extending radially outwardfrom spindle 82 toward bore 77 and a further thin quarter-wavelengthcylindrical sleeve 94 rightwardly extending therefrom is provided onspindle 82 between unit 88, 89 and the threaded portion 83. The secondsleeve 94 cooperates with bore 77 as a low characteristic impedancequarter-wavelength coaxial transmission line section, and with spindle82 as a higher characteristic impedance quarter-wavelength coaxialtransmission line section. An extremely low impedance is presentedbetween the periphery of end wall 88 and the bore 77, fully as effectiveas a direct connection therebetween but free from the troubles which canbe introduced by reliance upon frictional contact arrangements. Thisextremely low impedance results from the cooperation of the fourquarter-wavelength transmission line sections, as will now be described.

The quarter-wavelength section 94, 82, being shortcircuited at its leftend, presents a very high impedance at its right end. This impedance,with the contact between threaded section 83 and plate 79 appearing asnegligible series impedance relative thereto, is transformed by theouter quarter-wavelength transmission line section 94, 77 to anextremely low impedance at its left-hand end. The nextquarter-wavelength transmission line section 89, 82, beingshort-circuited at its left-hand end, presents very high impedance atits right-hand end, and this very high impedance and the low impedanceat the left hand end of section 94, 77, acting in series, aretransformed to extremely low impedance at the radial gap at theperiphery of disc end wall 88. Throughout a reasonable tuning range ofthe wavemeter, this gap impedance is so low as not only to guard againstmicrowave energy leakage through the space to the right of wall 88 butalso to insure freedom from appreciable reaction of the wave trap systemupon the Q and tuning of the resonator.

Microwave energy is led into the high Q chamber within bore 77 throughwave guide 95, illustrated as a rectangular wave guide of thefundamental energy transmission mode with its transverse electric linesoriented vertically as seen in Fig. 1. This provides transverse electricmode excitation of the cavity resonator, with the transverse electriclines extending between the lower surface region and the upper surfaceregion of bore 77 (Figs. 1 and 2) as indicated by the E lines in Fig. 3,and with magnetic flux in horizontal loops (with reference to theorientation of Figs. 1 and 2) as indicated by the closed line loops inFig. 5 and by the distribution of the dots in Fig. 3.

The maximum electric field intensity is in the middle region of theinterior of bore 77, as viewed in Fig. 2, with lesser intensity in theleft and right side regions, the electric intensity distribution beingas shown in Fig. 4. The magnetic field intensity is maximum along theside regionsc of the bore 77 as viewed in Fig 2, as illustrated by thedistribution of dots in Fig. 3 and by the magnetic flux lines in Fig. 5,and as indicated by the graph in Fig. 6.

A continuous indication of 'the responsiveness of the cavity resonatormay readily be provided by a crystal detector and milliamcter, asindicated in Figs. 1 and 2. The crystal detector .is astandard miniatureunit with a ceramic tubular body section 43 containing the sensitiveelement, and with a major end ferrule 41 flanged as at 42 and a minorend ferrule including a pin 44. This crystal detector is inserted withina cartridge unit 44 comprising two principal metal tubular parts 46 and69 in threaded engagement, the latter having affixed thereto a metalbushing 71. which supports and aligns the ceramic body section 43 of thecrystal unit. The upper part of the bushing 71 is counterbored toprovide a slight clearance for flanged ferrule 41, 42, and to formtherewith an air-dielectric capacitor of the closely-spaced coaxialcylindrical surfaces.

A coupling loop 74 of stiff wire is attached to the end of part 69 ofthecartridge, and is provided with a cuplike contactor 73 for receivingpin 44. A solid dielectric body 48 is fitted within the metal shell part46 of the cartridge, and is provided with a fixed coaxial insert pin 47.A coaxial connector is thus formed at the ends of parts 46 and 47, forconnection of a suitable cable leading to the milliameterschematicallyindicated at St in Fig. 2. A spring 52 is attached to the opposite endof pin 47, and is arranged to act against the major ferrule 41, 42 of.the crystal unit to hold it firmly seated in the cartridge and to.provide electric connection between ferrule 41, 42 and pin 47.

As shown in Figs. 1 and 2, the end of the cartridge part 69 providedwiththe coupling loop 74 communicates with the bore 77, to the extentthat magnetic coupling is provided between loop 74 and the magnetic fluxin the resonator.

The apparatus as illustrated in Figs. 1 and 2 is placed in operation byconnecting a microwave energy source to the wave guide 95. Sleeve 84 isthen slowly revolved, and the piston endsurface 38 is thereby movedgradually along the axis of bore 77 to provide very gradual change ofthe length of the chamber 80 until chamber 80 is of such size as toafford resonance at the frequency of the energy supplied through waveguide 95 and aperture 96. The condition of resonance is indicated bymaximum deflection of the pointer of meter 50 from its zero-currentposition. The readings of scales 85 and 87 are readily associatedwith=resonance at known frequencies produced by a. frequency standardsystem, and a calibration chart or graph is made up for ready referencein use of the wavemeter in the field or laboratory, for ascertaining thefrequency (and wavelength) of microwave energy, or for enablingoscillators to be set to generate energy of a predetermined wavelength.

The wavemeter illustrated in Fig. 7 is very similar to that of Figs. 1and 2, but'is of larger dimensions and greater measurement range. Body97, which serves both as the cylindrical wall of the resonator and asthe stationary micrometer barrel, has a cylindrical bore 93 closed atopposite ends'by Walls 99 and 101.. Rotatable sleeve m2 is secured tothe outer end of a spindle 103 thrcadediy inserted in a central boss ofwall 101.

Cylindrical-shell hollow pistons 104 and 105 are provided on theleft-hand end of the spindle, with quarterwavelength transmission linedimensions included therein. these units being arranged to act in thesame manner as the corresponding parts of the wavemeter shown in Fig. 1.

Chamber 106 formed in the wavemeter of Fig. 7 is much larger thanchamber 80 of Fig. l, and is employed for determination of wavelengthsover a somewhat larger frequency range. The wavemeter of Fig. 7 isprovided with an output detector and meter arrangement (not shown)similar to that of Fig. l. is connected to end wall 99 of the wavemeterand a centrally located aperture is provided in the end wall foradmission of energy from the wave guide 109 into the chamber 106.

The operation ofthe wavemeter of Fig. 7 is substantially identical tothat of the wavemeter of Figs. 1 and 2.

In the wavemeters above described, a hollow piston system is providedwithin one end of a cylindrical cavity system, and arranged with suchradial and cylindrical conductive surfaces as'to prevent energy leakagebeyond the p1ston surface transversely bounding the cylindrical cavity,and providing extremely high Q in the resonator. A coax1al outputdetection system is provided for indicating the variation of theintensity level within the resonator, and a rprecision micrometer drivesystem is The wave guide 109 h 1;; provided for rotatably controllingthe longitudinal movement of .the hollow piston, .and permitting theobtaining of a high precision reading of the extension of the pistonwithin the resonator bore.

As many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description and shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A microwave wavemeter for a wave guide transmission system includinga cavity resonator comprising two cylindrical hollow conductive elementseach closed at one end and disposed in adjustable telescopic relation toprovide a closed cavity of adjustable length, the cylindrical portion ofthe inner one of the elements being substantially a quarter wavelengthlong at a frequency within the operating frequency .band of thewavemeter, means for introducing microwave energy from said wave guideinto said cavity, means for adjusting said telescopic relation of saidelements to establish standing microwaves in said resonator in responseto said microwave energy, means including a concentric line looselycoupled to said cavityto induce in saidline signal currents in responseto said standing waves, a detector connected in said line to rectifysaid signal currents to provide signal demodulation currents, andindicator means coupled to said line and responsive to said demodulationcurrents for indicating resonance of said cavity resonator to saidmicrowave energy.

2. A microwave wavemeter for a wave guide transmission system includinga cavity resonator comprising two complementarily threaded elements,each element having a hollow cylindrical portion, the respectiveportions being disposed in adjustable telescopic relation to provide aclosed cavity of adjustable length, the cylindrical. portion of theinner one of the elements being substantially a quarter wavelength longat a frequency within the operating frequency band of the wavemeter,means for introducing microwave energy into said resonator, means foradjusting said telescopic relation of said elements to establishstanding microwaves in said cavity in response to said microwave energy.means including a concentric lineloosely coupled to said cavity toinduce in said line signal currents in response to said standing waves,a detector connected in said line to rectify said signal currents toprovide signal demodulation currents, andindicator means coupled to saidline and responsive to said demodulation currents for indicatingresonance of said cavity resonator to said microwave energy.

3. Wavemeter apparatus comprising a first member having a cylindricalhollow space therein, a piston member cooperating with said first memberto form a resonant chamber in said hollow space, calibrated screwpositioning means for varying the displacement of said piston member insaid cylindrical hollow space and for indicating the displacement of thepiston, said piston member having an end surface forming a boundary ofsaidresonant chamber but free from contact at its edge with thecylindrical interior surface of said first member, said piston memberand said first member being in contact in a region remote from saidedge, and said piston member and said first member including a series ofat least two tandem quarter-wavelength transmission line sectionsinterposed between said piston member edge and said contact region loosecoupling means for introducing ultra high frequency energy into saidresonator, and means for indicating the intensity of ultra highfrequency energy insaid resonator.

4. Wavemeter apparatus as defined in claim 3 wherein said piston memberend surface is substantially planar and perpendicular to the axis of thecylindrical interior surface of said first member.

5. Wavemeter apparatus as defined in claim 3, wherein said piston memberis rigidly connected to said calibrated screw positioning means and isrotated therewith as the displacement of said piston member is varied byadjustmentof said screw positioning means.

6. Wavemeter apparatus as defined in claim 3, wherein said series oftandem quarter-wavclength transmission line sections comprises fourquarter-wavelength coaxial transmission line sections, alternate onesofsaid transmisslon line sections including the line section extendingfrom said piston member edge having an appreciably lower ratio betweenthe diameters of their inner and outer conductors than the ratio betweenthe diameters of the inner and outer conductors of the other two of saidtransmission line sections.

7. An ultra high frequency wavemeter for precise determination offrequency of energy within a band of frequencies, comprising a hollowconductive body defining a chamber, a conductive plunger rotatable andaxially movable within said chamber, screw-threaded calibrated meansincluding one screw element rigidly fixed to said plunger for rotatingsaid plunger and varying the axial position thereof within said chamber,said body having a conductive cylindrical interior wall surface and saidplunger having a conductive cylindrical exterior wall surface coaxiallyadjacent to and of slightly lesser diameter than said cylindricalinterior wall surface and forming therewith a close-spacedhigh-capacitance coaxial transmission line section, said conductivecylindrical wall surface of the plunger being substantially one-fourthwave length long at a frequency in said band and said interiorcylindrical wall surface of the chamber being appreciably longer andsaid conductive cylindrical wall surfaces being subjected to relativemovement angularly and longitudinally as said plunger is moved in saidchamber, means providing relatively high impedance between saidconductive cylindrical wall surfaces at one end of said onefourthwavelength surface for providing extremely low impedance between saidconductive cylindrical wall surfaces at the opposite end of saidone-fourth wavelength surface, said plunger and said body togethercomprising a high-Q resonator of resonant frequency determined by theposition of said plunger, the extremely low impedance provided betweensaid conductive cylindrical wall surfaces being included in theeffective boundary of said resonator, all walls of said chamber beinghighly conductive and said resonator being free from any substantialenergy dissipating matter and accordingly being critically responsive tofrequency of energy therein, and means for loosely coupling into saidresonator ultra high frequency energy for frequency determination.

8. An ultra high frequency wavemeter for precise determination offrequency of energy within a band of frequencies, comprising a hollowconductive body defining a chamber, a conductive plunger rotatable andaxially movable within said chamber, screw-threaded calibrated meansincluding one screw element rigidly fixed to said plunger for rotatingsaid plunger and varying the axial position thereof within said chamber,said body having a conductive cylindrical interior wall surface and saidplunger having a conductive cylindrical wall surface coaxially adjacentto and of slightly lesser diameter than said cylindrical interior wallsurface and forming therewith a close-spaced high-capacitancetransmission line section, said conductive cylindrical wall surface ofthe plunger being substantially one-fourth wavelength long at afrequency in said band and the interior cylindrical wall surface of thechamber being appreciably longer and said conductive cylindrical wallsurfaces being subjected to relative movement angularly andlongitudinally as said plunger is moved in said chamber, means providingrelatively high impedance between said conductive cylindrical wallsurfaces at one end of said one-fourth wavelength surface for providingextremely low impedance between said conductive cylindrical wallsurfaces at the opposite end of said one-fourth wavelength surface, saidplunger and said body together comprising a high-Q cylindrical cavityresonator of resonant frequency determined by the position of saidplunger, the extremely low impedance provided between said conductivecylindrical wall surfaces being included in the effective boundary ofsaid resonator, all walls of said chamber being highly conductive andsaid resonator being free from any substantial energy dissipating matterand accordingly being critically responsive to frequency of energytherein, means for loosely coupling said resonator to a microwave energysystem for determination of frequency of energy therein, and means forloosely coupling a detector to said resonator for indicating thevariation of intensity of energy therein as said plunger is adjusted.

9. An ultra high frequency wavemeter as defined in claim 8, wherein saidmeans providing relaitvely high impedance between said conductivecylindrical wall surfaces at one end of said one-fourth wavelengthsurface comprises a one-fourth wavelength transmission line sectioncoupled thereto at one end and effectively shortcircuited at itsopposite end.

10. A tunable resonator comprising a first hollow conductive cylindricalmember closed at both ends by conductive end walls, a rod memberextending through one of said end walls of said member, means foradjustably positioning one end of the rod member within said firstmember, a second hollow conductive cylindrical member telescopicallypositioned within the first member, the second member having aconductive end wall secured to the inner end of said rod member, the rodmember passing axially through the cylindrical portion of the secondmember, whereby two concentric sections of transmission lines are formedby the inner surface of the first hollow member and the outer surface ofthe second hollow member, and by the inner surface of the second hollowmember and the outer surface of the rod member, means for couplingmicrowave energy into and out of the cavity region within the firsthollow member between the end wall of said second hollow member and theend wall of said first hollow member opposite said one end wall throughwhich the rod member extends, the cylindrical portion of the secondhollow member being substantially a quarter wavelength long within theresonant frequency band of the tunable resonator.

References Cited in the file of. this patent UNITED STATES PATENTS2,106,713 Bowen Feb. 1, 1938 2,106,771 Southworth Feb. 1, 1938 2,245,138Zotta June 10, 1941 2,423,506 Landon July 8, 1947 2,439,388 Hansen Apr.13, 1948 2,444,041 Harrison June 29, 1948 2,503,256 Ginzton et al. Apr.11, 1950

